North-central Tunisia Research Articles

The Cenomanian–Turonian Boundary Event (CTBE) as recorded in the northern margin of Africa: palaeoceanography of the Oceanic Anoxic Event (OAE-2), North-Central Tunisia

ABSTRACT Samples spanning the Cenomanian–Turonian transition at Guern Halfaya Section of North-Central Tunisia were analysed for elemental and isotopic chemostratigraphic characterization of the OAE-2 global anoxic event. Comparison with other published sections of the same age was achieved. These geochemical analyses, which cover the local Bahloul Formation, add new insights into the palaeoceanographic evolution of the southern Tethyan margin during the Cenomanian–Turonian transition. The use of discrete redox-sensitive and primary productivity-sensitive trace element data coupled to isotopic and biostratigraphic events is shown as leading proxies for the determination of prevailing warming palaeoceanographic conditions, in terms of productivity and climate, during the OAE-2 in the northern margin of Africa. Such event is believed to have generated one among the most significant source rocks of the region.

Upper Campanian-lower Maastrichtian integrated carbon isotope stratigraphy and calcareous microplankton biostratigraphy of North-central Tunisia

A comprehensive stratigraphic framework is presented for the upper Campanian-lower Maastrichtian succession of the Elles section in North-central Tunisia. This has been done by applying an integrated approach including calcareous nannofossil and planktonic foraminiferal biostratigraphy in conjunction with carbon isotope stratigraphy. The studied interval spans from the Uniplanarius sissinghii (CC21) to Reinhardtites levis (CC24) calcareous nannofossil zones and their coeval planktonic foraminiferal zones from the Contusotruncana plummerae Zone to Planoglobulina acervulinoides Zone. Seven positive and negative δ13C events have been recognized and considered for high-resolution global correlation. Isochroneity and diachroneity of the microplankton bioevents regarding the carbon isotope stratigraphy were assessed. Some discrepancies were reported, and some of them denote actual diachronism. However, the impact of the different taxonomic concepts and/or environmental issues cannot be totally excluded. The Campanian/Maastrichtian boundary has been precisely positioned within the lowermost part of the El Haria Formation at the base of the δ13C event CMBc (72.15 ± 0.05 Ma). This level projects directly below the first occurrence of the planktonic foraminifer Gublerina cuvillieri and about 9.4 m below the last occurrence of the nannofossil Uniplanarius trifidus.

Characterization of Cenomanian-Turonian oxic facies in Tunisia: sequence stratigraphy and correlation of recorded bioevents

On the southwestern Tethyan Margin of Tunisia, the well-known anoxic black shale facies of the Cenomanian-Turonian transition contrasts with oxic fossiliferous carbonates, first characterized here as lateral equivalents. In terms of sequence stratigraphy, the analysis of four wisely sampled sections in Central and Southern Tunisia led to interpret these deposits as a transgressive interval (TST). This interval spans the Whiteinella archaeocretacea Zone of foraminifera and is capped with a glauconite-rich highly bioturbated maximum flooding surface (MFS). In the reference section of Oued Bahloul and the Kalaat Senan area of North-Central Tunisia, these deposits overlay a Shelf Margin Wedge made of conglomeratic and bioclastic limestones. In South-Central and Southern Tunisia, the TST is characterized by the onset of oxic facies relaying laminated carbonates with local emergence surfaces. The analysis of both oxic and anoxic facies from the Cenomanian-Turonian transition allows identifying five successive bioevent markers, known elsewhere within the Tethyan domain: the extinction of the foraminifera genus Rotalipora (or Thomasinella), the Heterohelix shift, the Whiteinella proliferation, the filament event, and the appearance of Helvetotruncana helvetica. Among these bioevents, the Heterohelix shift coincides with the transgressive surface, while the filament event announces the maximal flooding surface. These Cenomanian-Turonian transition bioevents are of a particular relevance for regional and long-distance high-resolution correlations.

Ammonite stratigraphy and the belemnite genus Hibolithes in the higher Serdj Formation (Aptian-Albian boundary interval) in north central Tunisia

Ammonite stratigraphy and the belemnite genus Hibolithes in the higher Serdj Formation (Aptian-Albian boundary interval) in north central Tunisia

Organic matter and metal contents within the Cretaceous rocks of the Slata-Guern Halfaya area, North-Central Tunisia: Implication for ore genesis

The proximity of ore deposits, in the Slata-Guern Halfaya area (north-central Tunisia), to the Cretaceous Fahdene and Bahloul Formations raises the question of the role that these formations played in the emplacement of the ore. These formations are rich in organic matter (OM) with total organic carbon (TOC) values in the range of 0.2–1.4% and 0.62–5.46%, respectively, for the Albian–Vraconian facies of the Fahdene Formation at Slata and the Cenomano–Turonian facies of the Bahloul Formation at Guern Halfaya. Rock-Eval pyrolysis data indicates that the OM in the Fahdene Formation at Slata is relatively mature (oil window, 431 °C < Tmax < 447 °C) and with variable Production Index (PI, %)). Conversely, the OM preserved in the Bahloul Formation at Guern-Halfaya is immature (423 °C < Tmax < 433 °C). The analysis of whole rocks and the LA-ICP-MS analysis of inorganic fraction reveal high concentrations of trace elements (e.g., Pb, Zn) in both formations. We infer that seawaters were enriched in trace elements during the deposition of both formations. These trace elements were then sequestered by both organic and inorganic matter. Given the similar OM-rich lithologies (carbonates and marls) of these formations, the difference in trace element concentrations can be due to their contrasting thermal maturities. Concentrations of trace elements (Pb, Zn, Cd, and Ni) moderately correlate with OM content (TOC, pyrolyzable hydrocarbons and residual OM) positively in the organically-immature Guern-Halfaya ore district and negatively in the organically-mature Slata mining district. Subsidence of the OM-rich Cretaceous Formations (Fahdene and Bahloul), located in the deep part of the basin, resulted in thermal cracking of kerogen in these formations. This catagenesis allowed the dissociation of organo-metal ligands and subsequent expulsion of metals in low viscosity hydrocarbons during the Alpine orogeny. Metals and hydrocarbons migrated together from the subsiding areas of the basin and ultimately encountered salt diapirs. The mixing between metal-rich hydrocarbons and ore-bearing associated fluids, on the one hand, and the Triassic-derived sulfate-rich fluids, on the other hand, led to the precipitation of sulfides and sulfates at different times. A potentially effective trilogy to be targeted for base metal exploration is the juxtaposition of (i) mature OM-rich formations – potential sources of metals and hydrocarbons – with (ii) diapiric evaporites that served as a source of sulfur and sulfate, and (iii) a permeable reservoir that acted as a suitable site for ore precipitation.

Upper Cretaceous (Coniacian-Campanian) lithostratigraphy on the Saharan platform, Dahar Plateau, southern Tunisia

Four measured sections along the northern escarpment of the Dahar Plateau in southern Tunisia provide details of Cretaceous (Coniacian-Campanian) stratigraphy in this region of North Africa. These sections reveal various units of limestone with cross-stratification, limestone with in situ rudists, mud-supported limestone (carbonate mudstone, wackestone), argillaceous limestone, and limestone with stromatolites. These units are arranged vertically into several sequences of finer-grained limestone (mud-supported limestone, argillaceous limestone) overlain by coarser-grained limestone (limestone with cross-stratification, rudists, and/or stromatolites). Each sequence is interpreted as a shallowing upwards progression on a carbonate platform. Furthermore, cross-strqtified carbonate and beds with in situ rudists are more common in the lower parts of the measured sections, whereas units with stromatolites, argillaceous limestone, and finer-grained limestone are more common in the upper parts of the measured sections. The overall transition from the coarser-grained (higher energy) strata to the overlying finer-grained (lower energy) argillaceous strata is interpreted as having been caused by a relative fall in sea level that brought argillaceous sediment into the area. It is possible that the arrival of argillaceous sediments and the relative fall in sea level may have caused the demise of the rudist reefs. The lower interval (in which cross-stratified carbonates and in situ rudists are more common) is assigned to the Berressef Formation, whereas the upper interval (in which stromatolites, argillaceous limestone, and other fine-grained strata are more common) is assigned to the Toual Formation. Previous work suggests that the Berressef Formation is of Coniacian? to Santonian age, whereas the Toual Formation is of Campanian age. A regional cross section correlates these Cretaceous units on the Dahar Plateau with (1) rudist-bearing strata that are mapped in north–central Tunisia as the Merfeg Formation, and (2) argillaceous and marly limestone that is mapped in central Tunisia as the middle and lower Berda Formation and the middle Abiod Formation.

Migration pattern of hepatitis A virus genotype IA in North-Central Tunisia.

BackgroundHepatitis A virus (HAV) epidemiology in Tunisia has changed from high to intermediate endemicity in the last decades. However, several outbreaks continue to occur. The last reported sequences from Tunisian HAV strains date back to 2006. In order to provide an updated overview of the strains currently circulating in Tunisia, a large-scale molecular analysis of samples from hepatitis A cases was performed, the first in Tunisia.ResultsBiological samples were collected from patients with laboratory confirmed hepatitis A: 145 sera samples in Tunis, Monastir, Sousse and Kairouan from 2008 to 2013 and 45 stool samples in Mahdia in 2009. HAV isolates were characterised by nested RT-PCR (VP1/2A region) and sequencing. The sequences finally obtained from 81 samples showed 78 genotype IA and 3 genotype IB isolates.A Tunisian genotype IA sequence dataset, including both the 78 newly obtained IA sequences and 51 sequences retrieved from GenBank, was used for phylogenetic investigation, including analysis of migration pattern among six towns. Virus gene flow from Sfax and Monastir was directed to all other towns; in contrast, the gene flows from Sousse, Tunis, Mahdia and Kairouan were directed to three, two, one and no towns, respectively.ConclusionsSeveral different HAV strains co-circulate in Tunisia, but the predominant genotype still continues to be IA (78/81, 96% isolates). A complex gene flow (migration) of HAV genotype IA was observed, with Sfax and Monastir showing gene flows to all other investigated towns. This approach coupled to a wider sampling can prove useful to investigate the factors underlying the spread of HAV in Tunisia and, thus, to implement appropriate preventing measures.

Dynamic sedimentary conditions during periods of enhanced sequestration of organic carbon in the central southern Tethys at the onset of the Cenozoic global cooling

Stagnant bottom-water conditions (e.g., low and stable redox potential, long-water residence time) is an assumption commonly used to explain the preservation and burial of high amounts of organic carbon (Corg) in marine sediments. Rather than stagnant conditions, the evidence presented here from north-central Tunisia supports dynamic conditions during formation of variably Corg-rich, outermost shelf carbonates of the early–middle Eocene. The dynamic conditions are inferred by the deposition of four distinct lithofacies in this outermost shelf setting. Shedding of carbonate (i.e., mud and fragmented bioclasts) from the shallower source areas controlled the distribution of all lithofacies, with higher amounts of transported benthic debris occurring in the most proximal lithofacies and vice versa. This carbonate shedding also controlled the deposition of three orders of lithological cycles, from limestone/marly limestone couplets grading to cycles made up of groups of couplets. Bottom-water redox potential varied in intensity throughout this depositional setting, with moderate oxygen depletion (suboxic conditions) in the southern sector of north-central Tunisia and much higher oxygenation in the northern area. Evidence for suboxic bottom waters in the southern sector (higher Corg contents) is provided by higher trace metal (Cu, Ni, Zn, Cr, Mo, U and V) enrichments than in the northern area. Regionally heterogeneous primary productivity of surface waters is suggested to have caused a higher Corg burial flux in the southern sector compared to the north, a situation interpreted to have been related to varying upwelling patterns due to the effects of regional palaeogeography and the dominant wind patterns.The deposition of the studied Corg-rich carbonates spanned part of the calcareous nannofossil Zones NP13 to NP14 (~50–48m.y. ago) and coincided with the initiation of the Cenozoic global cooling subsequent to the early Eocene climatic optimum (EECO) (~52–50m.y. ago). An implication is that an increased Corg burial in north-central Tunisia could have been part of a major event sequestering atmospheric CO2 in marine sediments that caused climatic cooling immediately after the EECO. This implication, however, is difficult to reconcile with the data available elsewhere. Other than our Tunisian carbonates, major Corg sequestration in marine sediments of the early–middle Eocene transition are poorly documented globally. If this scenario is confirmed, other negative feedbacks, such as enhanced continental weathering, increased terrestrial carbon stock, decreased CO2 outgassing and/or changes in ocean circulation, would have been more influential to the onset of the Cenozoic global cooling.

In search of the Latest Danian Event in a paleobathymetric transect off Kasserine Island, north-central Tunisia

Danian (lower Paleocene) sequences in north-central Tunisia are dominantly composed of marls and shales but a conspicuous, indurated glauconite-bearing marker bed is associated with the P3a–P3b transition. This glauconite bed is considered to correlate with the Latest Danian Event (LDE) described from the Nile Basin in Egypt, with the ‘top Chron C27n event’ (Atlantic and Pacific Oceans) and with the ‘CIE-DS1’ (Zumaia, Spain). The LDE is thought to reflect a short period of global warming, similar to the Paleocene Eocene Thermal Maximum, but of lesser magnitude. The presence of a glauconitic bed at the P3a/P3b transition in Tunisia suggests that the sequence is condensed, which is confirmed by planktic foraminifer and nannofossil biostratigraphy, and by the absence of the typical LDE beds found in most Egyptian sections. Benthic foraminiferal assemblages were quantitatively analyzed in three sections in north-central Tunisia (Sidi Nasseur, Garn Halfaya, Elles) in order to characterize the paleoenvironmental evolution during the late Danian and compare this with sections in Egypt. The benthic foraminiferal assemblages indicate that the Tunisian sections were located along a depth- and a paleoenvironmental gradient on the shelf north of Kasserine Island. The reconstructed depth range is comparable with sections in Egypt, and encompasses middle neritic (Sidi Nasseur section) to shallow upper bathyal depths (Elles section, with up to 16% Gavelinella beccariiformis below the P3a/P3b). Like in Egypt, assemblage shifts across the P3a/P3b subzonal transition indicate shallowing and a transition to a more eutrophic paleoenvironment, characterized by relatively high abundances of buliminids and Stainforthia sp. The δ13C and δ18O records generated on well-preserved specimens of the ostracode species Bairdia failed to demonstrate the presence of the carbon isotope excursion (CIE) associated with the LDE at the base of Subzone P3b. A condensed section or a hiatus associated with the glauconite bed would explain why the CIE is not recorded in the Tunisian sections.

High resolution biostratigraphy of the Cenomanian–Turonian interval (OAE2) based on planktonic foraminiferal bioevents in North-Central Tunisia

High-resolution foraminiferal biostratigraphic analysis has been undertaken on the Bahloul Formation, in Bargou, Jerisa, Guern Halfaya (Central Tunisia), Kherij (North-eastern Tunisia) and Gafsa (southern Tunisia) sections, which prove the preservation of the worldwide expression of the Cenomanian–Turonian transition (C–T) Oceanic Anoxic Event (OAE2). This study confirms that the OAE2 was rather a long-term global event. The high resolution biostratigraphical analysis has allowed subdividing the Whiteinella archaeocretacea zone of the standard biostratigraphic scheme. The ability to apply same subzones in both Tethyan and transitional/Boreal provinces during the C/T boundary has important palaeobiogeographical and palaeoecological implications. The palaeoecological framework shows that the OAE2 development is composed of several successive phases. In addition, a faunal turnover during the W. archaeocretacea zone interval is recorded. However, we confirm that the faunal turnover in the W. archeocretacea zone did not result in a loss of biodiversity, but rather in replacement through originations.

Application of Facies Associations, Integrated Prediction Error Filter Analysis, and Chemostratigraphy to the Organic-Rich and Siliceous Cenomanian-Turonian Sequence, Bargou Area, Tunisia: Integrated Sequence Stratigraphic Analysis

Facies associations, integrated prediction error filter analysis (INPEFA) of spectral Gamma-ray data, Sr/Ca and Mn chemostratigraphy, and sequence stratigraphy of the organic-rich and siliceous Cenomanian-Turonian Bahloul formation have been studied in Bargou section, located in north-central Tunisia. The studied section is subdivided into seven facies evolving from platform to basin deposits. Based on basin geometry, facies distribution, spectral Gamma-ray INPEFA curves, Sr/Ca and Mn profiles patterns, and the sequence was also subdivided into shelf margin wedge (uppermost Fahden Formation-lowermost Bahloul), basal transgressive systems tract (remainder Bahloul Formation) and early high-stand systems tract (Kef Formation). The Sr/Ca ratios, Mn profile, sequence stratigraphic, and cyclostratigraphic interpretations reveal the existence of four transgressive parasequences deposited throughout 420 kyr within the organic rich and siliceous Bahloul facies.

Increased terrigenous influx but no drowning: palaeoenvironmental evolution of the Tunisian carbonate platform margin during the Late Aptian

During the Aptian, some carbonate platforms of the sub-tropical realm (for example, on the northern Tethys margin or in the Gulf of Mexico) were affected repeatedly by severe perturbations in the carbonate production factory and drowning, preferentially during global warming events such as the Early Aptian Oceanic Anoxic Event 1a and a prominent mid-Late Aptian warming interval. These platform growth crises have been explained mainly by strongly increased coastal runoff (for example, siliciclastics and nutrients) in combination with pronounced eustatic sea-level rises. In the last few years, increasing evidence suggests that carbonate platforms of lower latitudes were generally less or even not affected by environmental perturbations during these events. This raises the question as to the responsible factors that promoted platform growth or decline in different latitudinal areas. In this study, Upper Aptian (Middle Gargasian to Uppermost Clansayesian) inner-tropical carbonate ramp deposits of the Serdj Formation at Djebel Serdj, north-central Tunisia are studied in detail with regard to microfacies, lithology, biostratigraphy and chemostratigraphy. These data allow reconstruction of the palaeoenvironmental evolution of the Tunisian carbonate platform margin and investigation of its response to the prominent mid-Late Aptian warming interval. The unusually expanded, 600 m thick Serdj Formation consists of limestones, marlstones and siltstones, suggesting deposition within mid-ramp to inner-ramp palaeoenvironments. Deposits of the mid-Late Aptian are represented by quartz-rich platform carbonates and siltstones, probably resulting from increased coastal runoff on the Tunisian shelf as a response to global warming and accelerated water cycling. The siliciclastic input was accompanied by elevated nutrient levels as indicated by a partial decline in the abundance of oligotrophic biota and mass occurrences of orbitolines and green algae. Carbonate platform drowning during the mid-Late Aptian, as reported from the sub-tropical realm, has not been identified. A comparison with other tropical river-influenced platforms suggests that none of them drowned during the mid-Late Aptian. One important reason might be widespread arid to semi-arid climatic conditions within lower latitudes during that time, promoting platform growth due to comparably low nutrient runoff.

Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia

Thick Aptian deposits in north central Tunisia comprise hemipelagic lower Aptian, reflecting the sea-level rise of OAE 1a, and an upper Aptian shallow marine environment characterized by the establishment of a carbonate platform facies. Carbon stable isotope data permit recognition of the OAE 1a event in the Djebel Serdj section. Cephalopods are rare throughout these successions, but occurrences are sufficient to date the facies changes and the position of the OAE1a event. Ammonite genera include lower Aptian Deshayesites, Dufrenoyia, Pseudohaploceras, Toxoceratoides and ? Ancyloceras; and upper Aptian Zuercherella, Riedelites and Parahoplites. Correlation of carbon isotope data with those of other Tethyan sections is undertaken together with the integration of planktonic foraminiferal data.

Microfacies, biostratigraphy, and geochemistry of the hemipelagic Barremian–Aptian in north-central Tunisia: Influence of the OAE 1a on the southern Tethys margin

Upper Barremian–lower Upper Aptian hemipelagic deposits of the Hamada Formation in the Djebel Serdj area, north-central Tunisia were studied in detail with regard to microfacies, biostratigraphy, δ 13C stratigraphy, and geochemistry. Our data provide insights into the palaeoenvironmental evolution and sea-level fluctuations of the Tunisian shelf. The unusually expanded deposits consist of mud-, wacke-, and packstones which reflect mid- and outer-ramp depositional environments. Planktonic foraminifer and δ 13C stratigraphy allowed us to establish a detailed time-frame and the recognition of the Lower Aptian Oceanic Anoxic Event 1a (OAE 1a) as well as time-equivalent deposits of shallow-marine carbonate-platform drowning. Based on our microfacies studies, we subdivide the studied sections into four genetic intervals: a pre-OAE 1a interval, an OAE 1a and platform-drowning-equivalent interval, and a post-platform-drowning interval. We present a 3rd-order sea-level curve for the Tunisian shelf, deriving from the results of our microfacies studies. Deposits of the OAE 1a in the sections investigated are characterised by bioclastic wacke- and packstones with high abundances of poorly preserved radiolarians and moderately to well preserved planktonic foraminifers, suggesting a transgression and an eutrophication of the upper water column. Scarceness of benthic macrofossils, low abundances of small benthic foraminifers and ostracods possibly suggest dysoxic conditions at the seafloor. Mudstones of the platform-drowning time-equivalent deposits, directly overlying the OAE 1a, are partly showing a pronounced drop in carbonate content and are scarce of macrofossils.

Phylogenetic analysis of mtCR-1 sequences of Tunisian and Egyptian hares ( Lepus sp. or spp., Lagomorpha) with different coat colours

North African hares are currently considered belonging to cape hares ( Lepus capensis), except for an isolated occurrence of L. victoriae in NW Algeria. However, the few existing molecular data are not unequivocal. Here, we study sequence variation (415 bp) in the hypervariable domain-1 of the mitochondrial (mt) control region, of hares with different coat colour from north-central Tunisia and NW Egypt, to test Petter's [(1959): Eléments d’une révision des Lièvres africains du sous-genre Lepus. Mammalia 23, 41–67] hypothesis that North African hares belong to L. capensis. Seven Tunisian and one Egyptian haplotypes were revealed from 28 hares and compared phylogenetically to 245 haplotypes of various Lepus species downloaded from GenBank. Neighbour joining (NJ) and principal coordinate (PCO) analyses based on a Tamura-Nei 93 distance matrix, as well as maximum parsimony (MP) analysis concordantly grouped all currently obtained haplotypes together into one monophyletic clade, and revealed relatively close relationships to the clades of African scrub hares ( L. saxatilis) and brown hares ( L. europaeus). The three distinguished coat colour types of Tunisian hares were paralleled only to a small extent by sequence differentiation. Haplotypes of L. capensis from the nominal Cape province of South Africa, North Africa, and China clustered into different major clades, respectively, with Chinese L. capensis haplotypes forming only a subclade within a major clade that encompassed predominantly “mountain/arctic hare-type sequences” in addition to sequences of several other palaearctic and nearctic species. One further Chinese L. capensis haplotype clustered into the L. comus clade. These results indicated occurrence of introgression and/or shared ancestral polymorphism. Such an evolutionary scenario implies using nucelar markers in addition to mtDNA for phylogenetic inferences in the genus Lepus; nevertheless, mtDNA is still useful for inferring phylogenetic history and biogeography of hares.

‘A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs: Jebel Ousselat, northern Tunisia’ by E. Vennin et al .

Simon J. Beavington-Penney and Andrew J. Barnett write: Vennin et al . (2003) have provided an interesting study of the Ypresian (Eocene) El Garia Formation of Tunisia. This unit has been the subject of numerous studies, largely because it reservoirs hydrocarbons within the Gulf of Gabes (e.g. Loucks et al . 1998; Racey et al . 2001; Jorry et al . 2003). Because of the economic significance of this unit, its correct interpretation, especially in terms of the continuity and correlation of depositional sequences (and, therefore, reservoir layering) is crucial. Using the well-studied outcrops to the west of Kairouan in north-central Tunisia, Vennin et al . (2003) present a sequence stratigraphic model of the El Garia Fm. Their high-resolution model recognizes three orders of depositional sequences (third to fifth), with their ‘small-scale’ cycles occurring as individual decimetre- to metre-scale beds (which, they suggest, fall within the range of the duration of Milankovitch cycles) and groups of these beds which form metre- to decametre-scale, fifth-order packages. Vennin et al . (2003) claim that this hierarchy of cycles was produced by a nested hierarchy of eustatic sea-level change. We would like to suggest that these cycles (and the invoked causal sea-level change) are debatable, not least because they contain no unambiguous evidence of relative sea-level change (i.e. subtidal deposits capped directly by subaerial exposure surfaces). The El Garia Fm. was deposited over a period of approximately 2.8 Ma (Racey et al . 2001), during which time several third-order sea-level changes might be expected to have occurred. However, earlier studies from both outcrop and subsurface (offshore Tunisia borehole core) have not noted widespread surfaces that can be correlated within the El Garia Fm. (such as the Fe-rich hardgrounds which Vennin et al . (2003) record as bounding their fifth-order cycles) and have …

Palygorskite genesis through silicate transformation in Tunisian continental Eocene deposits

AbstractThe mineralogical and geochemical characteristics of Eocene continental sediments in south central Tunisia (Chebket Bouloufa and Jebel Hamri) and in north central Tunisia (Jebel Lessouda and Jebel Rhéouis), which contain considerable amounts of palygorskite, were studied. The clay fraction of the sediments also comprises illite, kaolinite, Mg smectite and Al smectite, together with carbonates (calcite and/or dolomite), quartz, gypsum and feldspars, all of which are present in extremely variable proportions.The textural characteristics of the samples containing most palygorskite, as well as the chemical composition of the fibres and the contents of certain trace and rare earth elements suggest that the genesis of this fibrous clay is intimately linked to the diagenetic transformation of illite, mixedlayered minerals and/or Al smectite, as has also been observed in contemporaneous deposits in Morocco.

Palygorskite genesis through silicate transformation in Tunisian continental Eocene deposits

The mineralogical and geochemical characteristics of Eocene continental sediments in south central Tunisia (Chebket Bouloufa and Jebel Hamri) and in north central Tunisia (Jebel Lessouda and Jebel Rhéouis), which contain considerable amounts of palygorskite, were studied. The clay fraction of the sediments also comprises illite, kaolinite, Mg smectite and Al smectite, together with carbonates (calcite and/or dolomite), quartz, gypsum and feldspars, all of which are present in extremely variable proportions.The textural characteristics of the samples containing most palygorskite, as well as the chemical composition of the fibres and the contents of certain trace and rare earth elements suggest that the genesis of this fibrous clay is intimately linked to the diagenetic transformation of illite, mixedlayered minerals and/or Al smectite, as has also been observed in contemporaneous deposits in Morocco.

High-resolution planktonic foraminiferal analysis from the Cretaceous–Tertiary boundary at Ain Settara (Tunisia): evidence of an extended mass extinction

The Ain Settara section, located in the Kalaat-Senan area of north–central Tunisia, spans the Cretaceous–Tertiary (K–T) boundary which is characterized by a red layer and a thin non-bioturbated boundary clay. Sediment accumulation across the K–T boundary at Ain Settara was probably interrupted by three short hiatuses and/or condensed sedimentation. The first hiatus occurs at the top of the CF1 Zone; the second hiatus/condensation occurred just below the boundary clay and the third hiatus at the P0/P1a boundary, in the earliest Danian. These hiatuses are marked by weak unconformities, bioturbation and sudden disappearances/appearances of species which are known to disappear/evolve sequentially in continuous sections. Quantitative high-resolution planktonic foraminiferal analysis across zones CF1(upper), P0 and P1a(1) reveals an extended and selective mass extinction. All 41% of the species which disappeared at or below the K–T boundary are rare to very rare and primarily ecologically specialized keeled deeper-dwelling tropical–subtropical forms ( Globotruncana, Globotruncanita, Gublerina, Planoglobulina, Rosita ( Contusotruncana), Racemiguembelina). Their combined relative abundance varies between 10% and 15% of the total population at the end of the Maastrichtian. The K–T crisis thus appears more catastrophic when viewed in tropical–subtropical assemblages and based on analysis of larger species (>200 μm) which preferentially includes the more specialized forms, though, in fact, the K–T mass extinction actually involved a relatively small part of the foraminiferal population in terms of relative abundance. The pattern of extinction and changes in dominant population at Ain Settara appear to be very similar to the planktonic foraminiferal turnover of the other north–central Tunisian sections (El Kef, Elles). The selective mass extinction pattern suggests that the catastrophic effects of the bolide impact superimposed those related to long-term environmental changes, such as variations in temperature, sea-level and associated water-mass changes.

La formation bahloul en tunisie centro-septentrionale: variations latérales, nouvelle datation et nouvelle interprétation en terme de stratigraphie séquentielle

From various geological traverses made in North-central Tunisia and northern Tunisia the Bahloul Formation (Burollet, 1956) is considered to be variable in thickness and facies. This formation is characterized essentially by its lithology and structure, consisting of alternating laminated and darkish micritic limestones interbedded with dark foliated marls which are rich in organic matter (mean T.O.C. 2.66%). At the type locality (Oued Bahloul), the Bahloul limestones are wackestones rich in pelagic fauna (Ammonites, Calcisphaerulidae, Rotalipora, Whiteinella, Dicarinella, etc). This formation overlies a massive 2 m thick limestone unit; the base of which is eroded, rich in phosphate and quartz grains. The same unit consists of wackestone-packstone carbonate rich in Calcisphaerulidae, with filaments and benthonic debris at the top of the formation. Thus, the base of this unit which covers the Fahdene Formation corresponds to an unconformity, shown by a rapid change in the depositional regime. The base of the overlying Kef Formation is also marked by an abrupt change in the depositional regime. Microfauna and microfacies analyses cofirm that the Bahloul Formation, at its type locality, began in the late Cenomanian. This is confirmed by Ammonites ( Calycoceras naviculare Mantell, with Eucalycoceras gothicum Kossmat, Mantelliceras villei Coquand) and ended in the early to middle Turonian. The richness and the pelagic nature of the fauna prove that the Bahloul Formation sediments were deposed in a continental-slope environment during a transgressive interval within the UZ-25 sequence (Vail et al., 1987).